1. Field of the Invention
The present invention relates to an acceleration sensor for detecting acceleration, which is used for toys, automobiles, aircrafts, portable terminals and the like, and particularly to an acceleration sensor that can be produced using a semiconductor technology.
2. Description of the Related Art
Acceleration sensors utilizing a change in physical quantity such as a piezo resistance effect and a change in electrostatic capacity have been developed and commercialized. These acceleration sensors can be widely used in various fields, but recently, such small-sized acceleration sensors as can detect the acceleration in multi-axial directions at one time with high sensitivity are demanded.
Since silicon single crystal becomes an ideal elastic body due to the extreme paucity of lattice defect and since a semiconductor process technology can be applied for it without large modification, much attention is paid to a piezo resistance effect type semiconductor acceleration sensor in which a thin elastic support portion is provided at a silicon single crystal substrate, and the stress applied to the thin elastic support portion is converted into an electric signal by a strain gauge, for example, a piezo resistance effect element, to be an output.
As a conventional triaxial acceleration sensor, there is the one disclosed in, for example, Japanese Laid-Open Patent No. 63-169078, and its plan view is shown in FIG. 13, and a sectional view taken along the line XIVxe2x80x94XIV in FIG. 13 is shown in FIG. 14, and a perspective view is shown in FIG. 15. The acceleration sensor 500 has elastic support arms 530 each of a beam structure, constituted by a thin portion of a silicon single crystal substrate. A mass portion 520 in a center, which is constituted by a thick portion of a silicon single crystal substrate, and a frame 510 in a periphery thereof are connected by the elastic support arms 530. A plurality of strain gauges 560 are formed in each axial direction on the elastic support arms 530.
An entire structure will be explained, referring to FIG. 13, FIG. 14 and FIG. 15. The sensor 500 has the mass portion 520 constituted by the thick portion of the silicon single crystal substrate, a frame 510 placed to surround the mass portion 520, and two pairs of elastic support arms 530 in a beam form, which are perpendicular to each other and each constituted by the thin portion of the silicon single crystal substrate to bridge the mass portion 520 and the frame 510. When the acceleration works, the mass portion moves in the frame to deform the elastic support arms, and thus the deformation is detected by the strain gauges provided on the elastic support arms to obtain the acceleration that works. The acceleration in an X-axis direction in FIG. 13 is measured by the four strain gauges 560 provided on the elastic support arms extending in the X-axis direction, and the acceleration in a Y-axis direction is measured by the four strain gauges 560 provided on the elastic support arms extending in the Y-axis direction. The acceleration in a Z-axis direction is measured by means of all the strain gauges 560. By making four L-shaped through-holes 550 in the silicon single crystal substrate having the size of the frame 510, the mass portion 520 in the center, the frame 510 in the periphery and the support arms 530 bridging them are formed, and by making the support arm portions thin, the acceleration sensor is constructed to be deformable and highly sensitive.
Although the acceleration in the Z-axis direction is detected or measured by both the strain gauges 560 that detect X-axis acceleration and the strain gauges 560 that detect Y-axis acceleration in the acceleration sensor 500 shown in FIGS. 13 through 15, it is preferable that a circuit detecting Z-axis acceleration is separated from a circuit detecting X-axis/Y-axis acceleration. In the co-pending patent application, Chinese Patent Application N/A (Feb. 12, 2003), European Patent Application 03002164.6 (Feb. 3, 2003), Korean Patent Application 10-2003-008738 (Feb. 12, 2003) and U.S. Ser. No. 10/357,408 (Feb. 4, 2003) filed by the same assignee based on Japanese Patent Application 2002-33696 of Feb. 12, 2002, strain gauges for detecting Z-axis acceleration are different from strain gauges for detecting X-axis acceleration, while the Z-axis strain gauges are located on elastic support arms in X-axis direction in the same way as X-axis strain gauges.
In FIG. 16, an acceleration sensor 600 has a mass portion 620 in a center, a thick frame 610 around it, and elastic support arms 631, 632, 633 and 634 for bridging the mass portion 620 and the thick frame 610. Since the elastic support arms 631, 632, 633 and 634 are thin, the mass portion deforms the elastic support arms when acceleration acts on the mass portion 620. Large deformation of each of the elastic support arms occurs to end portions of the elastic support arms, that is, connecting portions of an edge of a top surface of the mass portion and the elastic support arms, and connecting portions of inside edges of a top surface of the thick frame and the elastic support arms. In order to enhance the sensitivity of the acceleration sensor, strain gauges are attached at the portions of the elastic support arms, which are deformed most by the acceleration.
In the acceleration sensor 600 in FIG. 16, strain gauges 661, 662, 663 and 664 for detecting acceleration in the X-axis direction, and strain gauges 681, 682, 683 and 684 for detecting acceleration in the Z-axis direction are placed on the elastic support arms 631 and 633. It is generally known that there exists the relationship as shown in FIG. 17 between sensitivities of the X-axis strain gauge and the Z-axis strain gauge (output with respect to acceleration 1 G, and drive voltage 1 V). When the acceleration of 1 G in the X-axis direction acts on the mass portion, bending moment applied to the elastic support arm is proportional to a product of a distance from the top surface of the mass portion to a center of gravity of the mass portion by a mass of the mass portion. Since the bending moment is proportional to the distance and the mass, the sensitivity in the X-axis direction changes as a quadric function with respect to the thickness of the mass portion. On the other hand, when the acceleration of 1 G acts in the Z-axis direction, the bending moment applied to the elastic support arm is proportional to a product of length of the elastic support arm and mass of the mass portion. When the thickness of the mass portion is changed, the length of the elastic support arm does not change, but only the mass of the mass portion changes, and therefore the sensitivity of the Z-axis becomes a linear function.
When the acceleration sensor 600 shown in FIG. 16 is produced with use of a Si single crystal substrate which is generally used in semiconductor fabrication, thickness of the Si single crystal substrate is 625 xcexcm or 525 xcexcm, and therefore as can be seen from FIG. 17, the sensitivity of the Z-axis strain gauge becomes larger than that of the X-axis strain gauge. If the sensitivities of the Z-axis strain gauge and the X-axis strain gauge are about the same, the amplifiers having about the same output amplification factors can be used for the Z-axis strain gauge and the X-axis strain gauge. In order to make the sensitivity of the Z-axis strain gauge the same as that of the X-axis strain gauge, it is suitable to make the acceleration sensor 600 with use of the Si single crystal substrate of thickness of about 800 xcexcm, but such a thick Si single crystal substrate as this has to be especially prepared only for this acceleration sensor, and this increases the cost of the acceleration sensor.
Alternatively, it is theoretically possible to change piezo-properties by changing impurity concentrations of the piezoresistors used for the Z-axis strain gauge and X(Y)-axis strain gauge. However, when the piezoresistors are formed, only doping of the impurities to the Z-axis resistor has to be performed in a separate process step, and therefore the cost is raised. Further, there arises the fear that the thermal properties of the Z-axis piezoresistor and the X(Y)-axis piezoresistor differ.
Alternatively, it is possible to change the shape of the Z-axis resistor from that of the X(Y)-axis resistor to reduce the output power of the Z-axis resistor and thereby make it about the same as the output of the X(Y)-axis resistor, but the resistance value is changed to make it difficult to keep the bridge balance, and therefore it is desirable that all the resistors have the same shapes.
Therefore, it is an object of the present invention to provide a compact and thin acceleration sensor having very little sensitivity differences among three-axis resistors of X-axis, Y-axis and Z-axis with low manufacturing cost, while maintaining resistivities and thermal dependency in the same level among them.
An acceleration sensor according to the present invention comprises:
a mass portion provided in a center of the acceleration sensor and having a top surface;
a thick frame surrounding the mass portion with a predetermined distance from the mass portion and having a top surface;
a plurality of elastic support arms each extending (e.g., in X-axis direction) from an edge of the top surface of the mass portion, bridging the top surface edge of the mass portion and an inside edge of the top surface of the thick frame and hanging the mass portion inside of the thick frame;
two first strain gauges disposed on a top surface of each of the elastic support arms with a distance from each other along the elastic support arm extending (e.g., in X-axis direction), and extending in the direction of the elastic support arm extending (e.g., in X-axis direction); and
two second strain gauges disposed on the top surface of the elastic support arm having the two first strain gauges and with a distance different from the first strain gauge distance between the two second strain gauges along the elastic support arm (e.g., in X-axis direction), and extending in the direction of the elastic support arm extending (e.g., in X-axis direction),
the two first strain gauges detecting an acceleration in the direction of the elastic support arm extending (e.g., in X-axis direction) and the two second strain gauges detecting an acceleration in the direction (Z-axis direction) perpendicular to the top surface of the mass portion.
In the acceleration sensor as described above, one of the two first strain gauges may be disposed entirely on the top surface of the elastic support arm so that an end of the one of the first strain gauges is substantially located at the inside edge of the top surface of the thick frame, and the other of the two first strain gauges may be disposed entirely on the top surface of the elastic support arm so that an end of the other of the first strain gauges is substantially located at the top surface edge of the mass portion. One of the two second strain gauges is desirably disposed, bridging the top surface of the thick frame and the top surface of the elastic support arm so that one end of the one of the second strain gauges is located on the top surface of the elastic support arm and the other end of the one of the second strain gauges is located on the top surface of the thick frame. And, the other of the two second strain gauges is desirably disposed, bridging the top surface of the mass portion and the top surface of the elastic support arm so that one end of the other of the second strain gauges is located on the top surface of the mass portion and the other end of the other of the second strain gauges is located on the top surface of the elastic support arm.
In the acceleration sensor, the distance between the two second strain gauges is preferably longer by 0.4 to 1.2 times a length of the strain gauges than the distance between the two first strain gauges. And, it is more preferably that the distance between the two second strain gauges is longer by 0.6 to 1.0 times a length of the strain gauges than the distance between the two first strain gauges.
In the acceleration sensor, the two second strain gauges may be disposed entirely on the top surface of the elastic support arm so that all ends of the two second strain gauges are apart from ends of the elastic support arm. In the case, the distance between the two second strain gauges is desirably shorter by 1.0 to 1.8 times a length of the strain gauges than the distance between the two first strain gauges. It is more desirable that the distance between the two second strain gauges is shorter by 1.2 to 1.6 times a length of the strain gauges than the distance between the two first strain gauges.
An acceleration sensor according to the invention comprises:
a mass portion provided in a center of the acceleration sensor and having a top surface;
a thick frame surrounding the mass portion with a predetermined distance from the mass portion and having a top surface;
two first elastic support arms extending in parallel and in opposite directions to each other (e.g., in +X-axis and xe2x88x92X-axis directions) from opposite edges of the top surface of the mass portion, bridging the top surface edges of the mass portion and inside edges of the top surface of the thick frame and hanging the mass portion inside of the thick frame;
two first strain gauges disposed on a top surface of each of the first elastic support arms with a distance from each other along the first elastic support arm extending (e.g., in +X-axis/xe2x88x92X-axis direction), and extending in the direction of the first elastic support arm extending (e.g., in +X-axis/xe2x88x92X-axis direction),
one of the two first strain gauges disposed entirely on the top surface of the first elastic support arm so that an end of the one of the two first strain gauges is located substantially at the inside edge of the top surface of the thick frame, and the other of the two first strain gauges disposed entirely on the top surface of the first elastic support arm so that an end of the other of the two first strain gauges is located substantially at the top surface edge of the mass portion;
two second strain gauges disposed on the top surface of each of the first elastic support arms with a distance longer by 0.4 to 1.2 times a length of the strain gauges than the distance between the two first strain gauges along the first elastic support arms extending (e.g., in +X-axis/xe2x88x92X-axis direction), and extending in the direction of the first elastic support arm (e.g., in +X-axis/xe2x88x92X-axis direction),
one of the two second strain gauges disposed, bridging the top surface of the thick frame and the top surface of the first elastic support arm so that one end of the one of the two second strain gauges is located on the top surface of thick frame and the other end of the one of the two second strain gauges is located on the top surface of the first elastic support arm, and
the other of the two second strain gauges disposed, bridging the top surface of the mass portion and the top surface of the first elastic support arm so that one end of the other of the two second strain gauges is located on the top surface of the mass portion and the other end of the other of the two second strain gauges is located on the top surface of the first elastic support arm;
two second elastic support arms extending in parallel and in opposite directions to each other (e.g., in +Y-axis and xe2x88x92Y-axis directions) from other opposite edges of the top surface of the mass portion, bridging the other top surface edges of the mass portion and other inside edges of the top surface of the thick frame and hanging the mass portion inside of the thick frame; and
two third strain gauges disposed on a top surface of each of the second elastic support arms with a distance from each other along the second elastic support arm extending (e.g., in +Y-axis/xe2x88x92Y-axis direction), and extending in the direction of the second elastic support arm extending (e.g., in +Y-axis/xe2x88x92Y-axis direction),
one of the two third strain gauges disposed entirely on the top surface of the second elastic support arm so that an end of the one of the two third strain gauges is located substantially at the other inside edge of the top surface of the thick frame, and
the other of the two third strain gauges disposed entirely on the top surface of the second elastic support arm so that an end of the other of the two third strain gauges is located substantially at the top surface edge of the mass portion,
the two first strain gauges detecting an acceleration in the direction of the first elastic support arm extending (e.g., in X-axis direction), the two second strain gauges detecting an acceleration in the direction (Z-axis direction) perpendicular to the top surface of the mass portion and the two third strain gauges detecting an acceleration in the direction of the second elastic support arm extending (e.g., in Y-axis direction).
An acceleration sensor of the invention comprises:
a mass portion provided in a center of the acceleration sensor and having a top surface;
a thick frame surrounding the mass portion with a predetermined distance from the mass portion and having a top surface;
two first elastic support arms extending in parallel and in opposite directions to each other (e.g., in +X-axis and xe2x88x92X-axis directions)from opposite edges of the top surface of the mass portion, bridging the top surface edges of the mass portion and inside edges of the top surface of the thick frame and hanging the mass portion inside of the thick frame;
two first strain gauges disposed on a top surface of each of the first elastic support arms with a distance from each other along the first elastic support arm extending (e.g., in +X-axis/xe2x88x92X-axis direction), and extending in the direction of the first elastic support arm extending (e.g., in +X-axis/xe2x88x92X-axis direction),
one of the two first strain gauges disposed entirely on the top surface of the first elastic support arm so that an end of the one of the two first strain gauges is located substantially at the inside edge of the top surface of the thick frame, and the other of the two first strain gauges disposed entirely on the top surface of the first elastic support arm so that an end of the other of the two first strain gauges is located substantially at the top surface edge of the mass portion;
two second strain gauges disposed on the top surface of each of the first elastic support arms with a distance shorter by 1.0 to 1.8 times a length of the strain gauges than the distance between the two first strain gauges along the first elastic support arms extending (e.g., in +X-axis/xe2x88x92X-axis direction), and extending in the direction of the first elastic support arm (e.g., in +X-axis/xe2x88x92X-axis direction),
the two second strain gauges disposed entirely on the top surface of the first elastic support arm so that all ends of the two second strain gauges are apart from all ends of the top surface of the first elastic support arm;
two second elastic support arms extending in parallel and in opposite directions to each other (e.g., in +Y-axis and xe2x88x92Y-axis directions) from other opposite edges of the top surface of the mass portion, bridging the other top surface edges of the mass portion and other inside edges of the top surface of the thick frame and hanging the mass portion inside of the thick frame; and
two third strain gauges disposed on a top surface of each of the second elastic support arms with a distance from each other along the second elastic support arm extending (e.g., in +Y-axis/xe2x88x92Y-axis direction), and extending in the direction of the second elastic support arm extending (e.g., in +Y-axis/xe2x88x92Y-axis direction),
one of the two third strain gauges disposed entirely on the top surface of the second elastic support arm so that an end of the one of the two third strain gauges is located substantially at the other inside edge of the top surface of the thick frame, and
the other of the two third strain gauges disposed entirely on the top surface of the second elastic support arm so that an end of the other of the two third strain gauges is located substantially at the top surface edge of the mass portion,
the two first strain gauges detecting an acceleration in the direction of the first elastic support arm extending (e.g., in X-axis direction), the two second strain gauges detecting an acceleration in the direction (Z-axis direction)perpendicular to the top surface of the mass portion and the two third strain gauges detecting an acceleration in the direction of the second elastic support arm extending (e.g., in Y-axis direction).
An acceleration sensor of the invention comprises:
a mass portion provided in a center of the acceleration sensor and having a top surface;
a thick frame surrounding the mass portion with a predetermined distance from the mass portion and having a top surface;
a plurality of elastic support arms each extending (e.g., in X-axis direction) from an edge of the top surface of the mass portion, bridging the top surface edge of the mass portion and an inside edge of the top surface of the thick frame and, hanging the mass portion inside of the thick frame;
two first strain gauges disposed on a top surface of each of the elastic support arms with a distance from each other along the elastic support arm extending (e.g., in X-axis direction), and extending in the direction of the elastic support arm extending (e.g., in X-axis direction); and
two second strain gauges disposed on the top surface of the elastic support arm having the two first strain gauges and at an angle with the direction of the elastic support arm extending (e.g., X-axis),
the two first strain gauges detecting an acceleration in the direction of the elastic support arm extending (e.g., in X-axis direction) and the two second strain gauges detecting an acceleration in the direction (Z-axis direction) perpendicular to the top surface of the mass portion.
In the acceleration sensor as described above, one of the two first strain gauges and one of the two second strain gauges may be disposed entirely on the top surface of the elastic support arm so that an end of each of the one of the two first strain gauges and the one of the two second strain gauges is substantially located at the inside edge of the top surface of the thick frame, and the other of the two first strain gauges and the other of the two second strain gauges may be disposed entirely on the top surface of the elastic support arm so that an end of each of the other of the two first strain gauges and the other of the two second strain gauges is substantially located at the top surface edge of the mass portion. It is preferable that each of the two second strain gauges is disposed at an angle of 10 to 30 degrees or 65 to 90 degrees with the direction of the elastic support arm extending (e.g., X-axis).
An acceleration sensor of the invention comprises:
a mass portion provided in a center of the acceleration sensor and having a top surface;
a thick frame surrounding the mass portion with a predetermined distance from the mass portion and having a top surface;
two first elastic support arms extending in parallel and in opposite directions to each other (e.g., in +X-axis and xe2x88x92X-axis directions) from opposite edges of the top surface of the mass portion, bridging the top surface edges of the mass portion and inside edges of the top surface of the thick frame and hanging the mass portion inside of the thick frame;
two first strain gauges disposed on a top surface of each of the first elastic support arms with a distance from each other along the first elastic support arm extending (e.g., in X-axis direction), and extending in the direction of the first elastic support arm extending (e.g., in X-axis direction),
two second strain gauges disposed on the top surface of each of the first elastic support arms at an angle of 10 to 30 degrees or 65 to 90 degrees with the direction (e.g., X-axis) of the first elastic support arm extending,
one of the two first strain gauges and one of the two second strain gauges disposed entirely on the top surface of the first elastic support arm so that an end of each of the one of the two first strain gauges and the one of the two second strain gauges is located substantially at the inside edge of the top surface of the thick frame, and
the other of the two first strain gauges and the other of the two second strain gauges disposed entirely on the top surface of the first elastic support arm so that an end of each of the other of the two first strain gauges and the other of the two second strain gauges is located substantially at the top surface edge of the mass portion;
two second elastic support arms extending in parallel and in opposite directions to each other (e.g., in +Y-axis and xe2x88x92Y-axis directions)from other opposite edges of the top surface of the mass portion, bridging the other top surface edges of the mass portion and other inside edges of the top surface of the thick frame and hanging the mass portion inside of the thick frame; and
two third strain gauges disposed on a top surface of each of the second elastic support arms with a distance from each other along the second elastic support arm extending (e.g., in Y-axis direction), and extending in the direction of the second elastic support arm extending (e.g., in Y-axis direction),
one of the two third strain gauges disposed entirely on the top surface of the second elastic support arm so that an end of the one of the two third strain gauges is located substantially at the other inside edge of the top surface of the thick frame, and
the other of the two third strain gauges disposed entirely on the top surface of the second elastic support arm so that an end of the other of the two third strain gauges is located substantially at the top surface edge of the mass portion,
the two first strain gauges detecting an acceleration in the direction of the first elastic support arm extending (e.g., in X-axis direction), the two second strain gauges detecting an acceleration in the direction (Z-axis direction) perpendicular to the top surface of the mass portion and the two third strain gauges detecting an acceleration in the direction of the second elastic support arm extending (e.g., in Y-axis direction).
Further, an acceleration sensor of the invention comprises:
a mass portion provided in a center of the acceleration sensor and having a top surface;
a thick frame surrounding the mass portion with a predetermined distance from the mass portion and having a top surface;
a plurality of elastic support arms each extending (e.g., in X-axis direction) from an edge of the top surface of the mass portion, bridging the top surface edge of the mass portion and an inside edge of the top surface of the thick frame and hanging the mass portion inside of the thick frame; two first strain gauges disposed on a top surface of each of the elastic support arms with a distance from each other along the elastic support arm extending (e.g., in X-axis direction), and extending in the direction of the elastic support arm extending (e.g., in X-axis direction); and
two second strain gauges disposed on the top surface of the elastic support arm having the two first strain gauges and with a distance different from the first strain gauge distance between the two second strain gauges along the elastic support arm (e.g., in X-axis direction), and extending at an angle with the direction of the elastic support arm extending (e.g., X-axis),
the two first strain gauges detecting an acceleration in the direction of the elastic support arm extending (e.g., in X-axis direction) and the two second strain gauges detecting an acceleration in the direction (Z-axis direction) perpendicular to the top surface of the mass portion.
In the acceleration sensor above, it is preferable that one of the two first strain gauges and one of the two second strain gauges are disposed entirely on the top surface of the elastic support arm so that an end of each of the one of the two first strain gauges and the one of the two second strain gauges is substantially located at the inside edge of the top surface of the thick frame, and that the other of the two first strain gauges and the other of the two second strain gauges are disposed entirely on the top surface of the elastic support arm so that an end of each of the other of the two first strain gauges and the other of the two second strain gauges is substantially located at the top surface edge of the mass portion. It is preferable that each of the two second strain gauges is disposed at an angle of 10 to 30 degrees or 65 to 90 degrees with the direction of the elastic support arm extending (e.g., X-axis). The distance between the two second strain gauges is preferably longer by 0.4 to 1.2 times a length of the strain gauges than the distance between the two first strain gauges. The distance between the two second strain gauges is more preferably longer by 0.6 to 1.0 times a length of the strain gauges than the distance between the two first strain gauges. Alternatively, the distance between the two second strain gauges is preferably shorter by 1.0 to 1.8 times a length of the strain gauges than the distance between the two first strain gauges. It is more preferable that the distance between the two second strain gauges is shorter by 1.2 to 1.6 times a length of the strain gauges than the distance between the two first strain gauges.